1. Field of the Invention
The invention relates generally to an equalization technique for data communication, and more particularly to an adaptive equalization technique using twice sampled non-return to zero (NRZ) data.
2. Background Art
In data communication, often the channel through which information is being sent limits the achievable bandwidth, or rate at which information may pass through the channel. One significant limitation that a channel can introduce is inter-symbol interference (ISI). ISI occurs when a portion of a signal representative of one bit of information interferes with a different portion of the signal representative of a different bit of information. Normally, the portions of the signal representative of different bits occur at different times. However, the limited bandwidth of a medium used to provide the channel may cause a NRZ pulse sent by a transmitter to be smeared out over multiple bit periods before it reaches a receiver.
Besides attenuating the maximum energy contained in any one bit period, the smeared pulses interfere with adjacent bits. Since adjacent bits are not correlated (or are only loosely correlated), the interfering information greatly raises the effective noise power and thus reduces the signal to noise ratio. In metallic wiring used for communication (such as the popular CAT-5 twisted pair cables used for ethernet, or printed circuit board microstrip traces used to interconnect chips on a board) a typical cause of ISI is a combination of frequency dependent attenuation and group delay. These frequency dependent properties are a result of skin-effect increase in resistance and frequency dependent insulator losses. If the attenuation in the channel varies by 6 dB or more over the range of frequencies contained in the transmitted signal, then, for some bit periods, the noise will even become greater in amplitude than the bit being sent, making it impossible to receive the bit without some form of equalization.
Equalization involves altering a signal so that it may be more easily received. A signal may be altered at the transmitter so that the influence of the channel on the signal will yield a signal capable of being properly recognized and received at the receiver. However, transmitter-based equalization is difficult since the transmitter must have a priori knowledge of the characteristics of the transmission channel and any changes that may occur to the characteristics of the transmission channel over time.
Equalization may also be performed at the receiver. Receiver-based equalization can use properties of the receive signal to adjust equalization parameters. However, receiver-based equalization becomes difficult if the signal is greatly distorted. As a result, receiver-based equalization has typically been applied only to relatively low bandwidth communication systems.
Adaptive equalization attempts apply a correct amount of equalization to the channel. However, determining the optimum equalization parameters may be prohibitively expensive or difficult. For example, while measurement of signal power is a simple technique to perform, signal power provides only an average of the attenuation from multiple frequencies. This averaging oversimplifies the problem, thereby limiting the accuracy of this technique. Moreover, this technique also requires a priori knowledge of the transmitted power which may not be possible to know (for example, a fiber channel allows for a range of transmit powers). Adaptive equalization may also introduce temporal inaccuracies if it is not continually adapted. For instance, a scheme where the adapting is performed only initially, or periodically, misses the temporal changes (for instance, those caused by humidity, power supply, or temperature changes) that may affect the amount of equalization needed at other times.